It is known that cements which are ground in a closed circuit with a material bed roller mill, disagglomerator and classifier or in roller mills do not correspond as regards their quality characteristics to those cements produced in ball mills. The same applies to blast furnace dust as well as to products constituting mixtures of these components. The differences in quality are demonstrated first and foremost in the addition of a higher quantity of water by contrast with ball mill products in order to achieve the standard stiffness of the mortar. This quantity is known from experience to be a measurement of the water requirement of the concrete in order to achieve a specific consistency. An increased water requirement of the concrete corresponds to an increased addition of water for the standard stiffness. Thus in the case of concrete produced with products from material bed roller mills or bowl roller mills it was necessary in the past to set a high water-cement ratio in order to achieve equal workability. The consequence is a higher pore volume and consequently a lower strength.
The said differences in quality are for the most part based in the narrower particle spectrum which products from material bed roller mills or bowl roller mills have in contrast to ball mill products. In the known grinding apparatus with material bed roller mills or bowl roller mills the classification stage usually occurs immediately after the material for grinding has passed once through the mill. This fulfils in an ideal manner the requirement of the size reduction theory that in the interest of optimum utilisation of energy the fines produced should be removed from circulation as quickly as possible. Accordingly, in the case of grinding apparatus in this category much less fine material is contained in the material for grinding which is delivered to the classifier than is the case in ball mill apparatus.
The particle spectrum of cement and cement-like products is usually represented as a sum distribution in the "RRSB grid" developed by Rosin, Rammler et al. The axis scales of this grid are chosen so that the sum distributions or normal mineral crushing products appear as straight lines. These sum distributions are described by two parameters:
the particle size d' for a specific screen residue (36.8%)
and the inclination n which corresponds to the tangent of the angle between the particle size line and the abscissa.
The basis for this method of representation is the empirical fact that the particle size distributions of very many mineral crushing products have a similar structure irrespective of their fineness.
The usual measure for the fineness of cement, blast furnace dust and similar products is the specific surface area or Blaine fineness. The higher this fineness is, the higher the strengths are of the mortars and concretes produced therefrom. The specific surface area is inversely proportional to the mean particle size (in the case of the same specific surface area the strength defined in the relevant standards is higher the narrower the product particle spectrum is).
The fewer fines are contained in the feed material for the classifier, the finer their separation limit must be set (see below) so that the product has the desired specific surface area. In order to compensate for the lack of fines a corresponding proportion of tailings must be separated off. Thus the product particle spectrum is narrowed from both sides. As a result the porosity of the product increases, and with the porosity the water requirement in order to achieve a specific mortar or concrete consistency also increases.
The connection explained above has been observed with the introduction of selective separators in ball mills and has led in some cases to the rejection of products.
The object of the invention, therefore, is to provide a method and apparatus for crushing material for grinding in such a way that the particle size distribution of the finished product can be set accurately over a sufficiently large range (and indeed--expressed as an alteration in the inclination n in the RRSB grid--by at least 0.2) in order to adapt the products of such energy-saving grinding apparatus to the standard of the products produced in the ball mill apparatus as regards their particle size distribution and thus also as regards the way they behave during processing and their strength development, and also in order to compensate for chemically induced (for example by the raw material) shortcomings by advantageous adjustment of the particle size distribution of the finished product.